When one eats hot dishes, they cool rapidly and it is desirable to heat them for keeping them at the good temperature. For this purpose, one has conceived chafing dishes which are heated electrically by Joule effect or by a gas capsule or candles. This system is relatively dangerous since the network voltage or a flame is brought on the table. Furthermore, that does not solve the problem of the cooling in the plates.
For the latter, a first solution, used mainly in restaurants, consists in serving in plates which have been beforehand heated. This necessitates an important organization and manipulations with risk of burns; anyhow, since the thermal capacity is low, the dishes cool rapidly before one has finished to eat them.
Another solution consists in electrically heating the plates when eating either by heating cushions which are placed under the plates or by heating resistors which are integrated in the plates. In the first case, there exists the risk of burns by the heating cushion when the plate is not in place and in the second case, the plate is heavy and connected to an electrical cable, what is not practical and can be harmful.
In the case of cafeterias, especially in works canteens, one is lead to eat the hot dish at first before it cools. Therefore, there are generally provided furniture's where the plate can be kept hot when eating the hors-d'oeuvres.
The patent DE 25 05 341 which describes in detail two realizations of vessels heated by induction, which are intended to be placed opposite to an inductor, is well known. The invention does not concern on the induction device.
The novelty consists in the realization of a conductive spiral on the bottom of the vessel for leading the induced currents and, especially, in the realization of a strap for closing the spiral.
Two methods of realization of this strap are detailed, one by insertion of a conducting wire in the material of the plate, the other by metallic deposit on the plate.
In this German patent, the inventor took pains to resolve a problem which does not exist. As a matter of fact, by heating a vessel through induction, in this vessel are involved currents which are the inverse image of the currents of the inductor. By replacing the spiral by four dosed concentric circles, the current in each circle is exactly the same as in the spiral, and also as in the inductive heating. It is not necessary to conceive a strap whatever the realized trick may be.
This document does not disclose a support (tray, set of table mats, et cetera) which is supplied with a complete feeding device comprising a voltage source and a chopper.
We know also two German utility models which are dealing with a subject which is close to our application.
These are the applications DE 296 12 514 of 9 7 96 and DE 296 21 541 of Dec. 12 1996.
The first model shows a tray comprising inductive elements, it is anterior to our application.
This tray with its inductor has to be connected to an adapted external element which generates harmonic current and is itself connected to a voltage source.
Here again, there is no support comprising an inductor, its feeding and its integrated voltage source.
The second model is posterior to our application.
The first model has been filed on July 9, 1996 but has been published only on December 12, 1996. It can not constitute an prior document that could be cited against our application. It is a prior right in Germany.
The basic problem of the invention is to provide a mobile heating device which allows, with simple means, to heat or to maintain at temperature a plate or a dish without risk of burns, without complicated manipulation, especially without having to undertake a connection, and without flame or electrical voltage which could be harmful.
Furthermore, the invention allows to adjust the temperature of the plate or the dish always without complicated manipulation.
For this purpose, the object of the invention is a mobile heating device of a cooking vessel, such as a plate or a dish, which is placed on a table, for keeping hot the hot foods contained in the said vessel, characterized in that it comprises:
a support of the vessel, such as a tray or a set of table mats, in which are arranged a supply module comprising a source of electric energy live with a safe direct voltage, a high frequency chopper which is fed by said source and at least a flat inductor of fine thickness supplied by the chopper and generating an electromagnetic field which is perpendicular to the plane of the table, and
a flat armature of fine thickness made up of completely closed turns and placed under the vessel parallel to the plan of the table,
the vessel being placed on the support and the armature being magnetically coupled with the inductor so that eddy currents flow in the armature.
The heating occurs only when the vessel is placed on the support and there is no heating, and therefore no risk of burn, when one puts hands on the set of table mats; in the same way, if one puts a metallic object, such as a fork or a spoon or a watch, on the support, there is practically no heating too.
In the case of cafeterias, it is no longer necessary to plate heating furniture's and it is possible to eat the meal in the normal order while having a hot dish at the good temperature.
There no particular manipulation since it is enough to put the plate on the support; especially, it is not necessary to realize an electric connection, the support remaining permanently connected.
Furthermore, the voltage of the support is a safe voltage, for example 12V.
The use of a high frequency allows to increase the efficiency of the coupling, therefore of the heating.
Advantageously, the inductor(s) have a high inductance at the frequency of the voltage generator and a low resistance and the armature has, seen from the primary a low inductance and a resistance which is substantially equal to the ratio of the square of the voltage of the source to the necessary power for the heating.
This disposition allows to obtain a maximum output for the heating.
According to another characteristic of the invention, the electromagnetic fields generated by the inductor and the turns of the armature are not rotational symmetrical so that the magnetic coupling of the armature with the inductor varies according to the relative angular position of the inductor and the armature.
Thanks to this variation of the coupling, one can adjust the temperature of the vessel.
According to an embodiment of the invention, the inductor and the armature are oval shaped.
According to another embodiment of the invention, the inductor comprises two substantially circular adjacent coils connected in series and generating electromagnetic fields of opposite directions and the armature comprises two adjacent sets of substantially circular turns.
This disposition allows also to obtain a variable coupling, but also to highly reduce the electromagnetic radiation of the inductor and, as a result, perturbations on the environment.
According to a first embodiment of the invention, the support of vessel is a tray and the source of electrical power is an electrochemical generator.
Advantageously, the electrochemical generator is a rechargeable element and the supply module comprises means for comparing the voltage on the inductor(s) and the voltage of the source and a negative feedback circuit which controls the pulsed voltage generator so that its duty factor vanes inversely to the voltage on the inductor(s).
This disposition allows to recharge the rechargeable element by induction between the supports.
According to another characteristic of the invention, the supply module is placed in an receptacle arranged along one edge of the tray and separated from the rest of the tray by a longitudinal recess.
It is therefore possible to superimpose trays by placing them head to foot for recharging the rechargeable elements.
Advantageously, the support of vessel comprises two inductors, a substantially centric one and one disposed in a corner.
This allows to keep the vessel hot by posing it in the corner when eating another dish, for example the hors-d'oeuvre.
According to a second embodiment of the invention, the support of vessel is a set of table mats, the source of electrical power is a rectifier circuit connected to a transformer fixed under the table and the supply module is placed in a part of the set of table which is folded under the table.
Advantageously, the transformer of the supply module is fixed under the table and the rest of the supply module is placed in a part of the set of table which is folded under the table.
This allows to simplify the manufacture, the totality of components, except the transformer, especially the chopper of current, being realized in the support.
Advantageously, the table comprises a common transformer feeding the totality of the sets of table placed on the table and the sets of table are fed by a cable lying under the table at its periphery.
This disposition allows to simplify the structure and to reduce the cost price. Moreover it allows to have a table with an arbitrary arrangement of the guests.
According to another embodiment of the invention, the inductor is integrated in the upper face of the table.
This disposition is particularly advantageous for restaurants, especially for the cafeterias.
Advantageously, the support of vessel comprises at least a projecting part which corresponds with an inductor and is used for the positioning of the vessel.
This allows to obtain automatically an optimal heating, because the centering of the plate is automatically realized, what insures an optimal coupling.
According to again another characteristic of the invention, the vessel comprises a thermometer, such as a LCD thermometer, on its upper face.
In this manner, one can adjust and check more precisely the temperature of the plate.